Spiral staircase



FIG. 1 is a front elevation of a spiral staircase displaying the face of lowest step;

FIG. 2 is a right side elevation showing the upper landing of the spiral staircase;

FIG. 3 is a left side elevation showing the back section of the spiral staircase;

FIG. 4 is a rear elevation of the spiral staircase 180° opposed to the view presented in FIG. 1;

FIG. 5 is a top plan view of the spiral staircase;

FIG. 6 is a bottom plan view of the spiral staircase;

FIG. 7 is a front perspective view of the spiral staircase. 

The ornamental design for a spiral staircase, substantially as shown and described. 